Yes, you can create a new HttpRequestMessage, set all the properties you need to, and then pass it to SendAsync.
var request = new HttpRequestMessage() {
RequestUri = new Uri("http://example.org"),
Method = HttpMethod.Post,
Content = new StringContent("Here is my content")
}
request.Headers.Accept.Add(...); // Set whatever headers you need to
var response = await client.SendAsync(request);